1. Field of the Invention
The present invention relates generally to computer-assisted design (CAD) systems, and in particular, to a method for cell selection during feature generation in a solid modeling system.
2. Description of the Related Art
Over the last decade, designers have changed their fundamental approach to graphics design, moving from 2D drawing systems to 3D solid modeling systems. New software makes solid modeling technology available and affordable to virtually anyone.
Solid modeling is a technique that allows designers to create dimensionally accurate 3D solid models in 3D space represented within a computer, rather than traditional 2D drawings. 3D solid models include significantly more engineering data than 2D drawings, including the volume, bounding surfaces, and edges of a design.
With the graphics capabilities of today""s computers, these 3D solid models may be viewed and manipulated on a monitor. In addition to providing better visualization, 3D solid models may be used to automatically produce 2D drawing views, and can be shared with manufacturing applications and the like.
Some 3D solid modeling systems generate parametric feature-based models. A parametric feature-based model is comprised of intelligent features, such as holes, fillets, chamfers etc. The geometry of the parametric feature-based model is defined by underlying mathematical relationships (i.e., parameters) rather than by simple unrelated dimensions, which makes them easier to modify. These systems preserve design intent and manage it after every change to the model.
Moreover, these features automatically change as the model is changed. The system computes any related changes to parts of the model that are dependent on a parameter, and automatically updates the entire model when the parameter is changed. For example, a through-hole will always completely go through a specified part, even if the part""s dimensions are changed to be bigger than the hole.
Two parametric features found in solid modeling systems are Rib and Web features. However, such features are not easily generated in a solid modeling system. Typically, a Rib or Web feature is usually created by sketching a single open profile. With the Rib, the open profile is converted to a closed profile by extending the ends of the open profile to intersect with a part and extend well beyond it, and the closed profile is extruded by a specified thickness in a perpendicular direction to the plane of the profile to generate a tool. With the Web, the open profile is converted to a closed profile by offsetting the open profile by the specified thickness and extending to intersect with a part in the plane of the profile. The closed profile is extruded by a distance to go well beyond the part. The tool and the part are then unioned, subtracted, and/or intersected in a non-regularized manner. This is followed by selecting pieces to create either the Rib or Web feature.
In current systems, the specification of the open profile and thickness is often limited. For example, one needs to align the ends of the profile carefully with respect to a part. Besides, the thickness can be specified either in the plane of the profile or in a plane perpendicular to the profile. Moreover, the use of geometric comparisons or analyses, such as point-in-cell or curve-in-cell queries, to identify pieces to keep are computationally expensive and ambiguous, and are subject to errors arising from numerical approximations.
Consequently, there is a need in the art for a solid modeling system that more efficiently and effectively generates Rib or Web features.
To address the requirements described above, the present invention discloses a computer-implemented solid modeling system that generates Rib and Web features. One or more attribute values are set to indicate whether extended start and end segments for each chain of an open profile modeled in the system intersect with a part modeled in the system. The open profile and the sheet with attributes and specified behaviors are inserted into the tool. The tool, which is generated from the open profile, is united with the part to create a cellular topology graph structure, wherein the attribute values get propagated to one or more cells of the cellular topology graph structure. The Rib or Web feature is then generated from one or more cells selected from the cellular topology graph structure based on the propagated attribute values.